Rig time is expensive and is always the subject of efforts to minimize it. One way to do this when running tools that wear out in use is to run in with a backup tool that can be deployed to finish the job should the primary tool either wear out or experience some other operational difficulty. For example in section milling where a length of a tubular is to be milled away to facilitate a lateral exit from a borehole the mills typically have a series of pivoting blades that are held retracted for running in and powered out against the casing or other tubular. Milling can occur in an uphole or a downhole direction depending on blade orientation. The simpler of these devices are pressure actuated so that when flow is initiated an internal piston is pushed whose movement releases a retraction tab on the blades that was employed for running in and another portion of the internal piston pushes the blades from behind against the surrounding tubular. Flow continues out of the tool to remove cuttings from the blades as the assembly is rotated to remove the surrounding tubular. The internal piston using the flow through the tool maintains a contact force on the surrounding tubular as the blades pivot as the milling progresses. A return spring takes over when flow is cut off to again allow the blades to retract to the point where they can be held retracted for tool removal.
If a single tool is run the surface personnel who monitor the milling rate will know from experience that the blades have worn and depending on the progress of the milling at that time it may mean that the tool has to be pulled out of the hole (POOH) and the blades replaced. This is a time consuming process and expensive for the operator. Having a spare mill in the hole would solve this problem but creates another problem. That problem is how to sequentially operate a primary and secondary mill that feature blades extending in response to pressure. The preferred operating method is to run the first mill until it wears and then retract its blades and finish the job with the second mill. The problem is that the same pressure that operates the first mill with extend the blades of the backup mill prematurely. Others skilled in the art have attempted to solve this problem but have failed to do it in a reasonably cost effective manner. Instead they have resorted to complex independent operating systems for the mills that use RFID tags and sensors to retract the blades of the spent mill and then to extend the blades of the second mill. This approach is shown in FIG. 13 of U.S. Pat. No. 8,141,634. The cost of this approach is prohibitive and the size of the tool is potentially increased to house the signal and power components which can make the design too large for use in some applications. Instead, the present invention continues to employ fluid pressure to extend blades but prevents the blades from the backup mill from extending with a lock on its piston actuator that is simply defeated with tool repositioning or simply just prevents actuation pressure that operates the primary mill from reaching the backup mill until the desired time for a switchover between mills. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.